The present invention relates to a method for charging secondary batteries, in particular, to decrease the charging time.
Since secondary batteries such as a lithium ion battery, have various advantages, such as a large capacity and so on, secondary batteries have been put to practical use in various fields. On the other hand, secondary batteries also have disadvantages, such as a small tolerance against overcharge and over-discharge, and a narrow usable voltage range. In view of this, a method of controlling the applied voltage for charging the batteries, in consideration of a voltage drop caused by the internal resistance of the batteries, has been effective. An example of this method has been disclosed in JP-A-7-240235 (1995). In accordance with the disclosed method, the charging current is switched off during the charging of a battery, the internal resistance of the battery is calculated based on values of the voltages before and after the switching off, and the voltage applied to the battery is maintained at a designated value by charging the battery with a charging voltage, to which is added an extra voltage equal to the voltage drop caused by the internal resistance of the battery.
A relationship between the charging voltage and the charging current at the time of charging a battery, when the battery is conventionally charged with a constant designated voltage without considering the internal resistance of the battery, is shown in FIG. 10. During the initial period, charging is performed with a constant current so as not to allow the charging current to exceed a designated value, because the voltage of the battery is low. After the voltage of the battery is increased to reach VU, the charging is performed with a constant voltage.
Then, the internal resistance of the battery during the charging is determined. An example of conventional methods for determining the internal resistance, wherein the internal resistance of the battery is determined by switching off the charging current completely during the charging of the battery, is indicated in FIG. 11. FIG. 11 shows that it takes a long time to stabilize the terminal voltage of the battery at a constant value after switching off the current. FIG. 11 also shows that it takes a long time to stabilize the voltage at a constant value after switching on the current.
In accordance with the above-described conventional method, the following problems are generated, because the charging current is switched off completely during the charging of the battery.
(1) It takes a long time to stabilize the voltage at a constant value after switching off the current.
(2) Accordingly, an effect of decreasing the charging time is not significant, because the charging is stopped for a long time during the charging operation.